The present invention relates to a fluid flow control valve for controlling the fluid pressure rising and lowering speed (flow rate to be exact), particularly a flow control valve suited for use in a fluid pressure control device such as an antilock brake control system and capable of continuously controlling the fluid flow when the valve is open and preventing the leakage of hydraulic fluid when the valve is deactivated, i.e. closed.
Antilock brake systems include, practically without exception, an electromagnetic valve as a fluid pressure control means. Even if the time period during which a pressure reduction or increase command is maintained is constant, the degree of pressure reduction or increase (that is, the flow rate at which hydraulic fluid flows through the valve) varies with the voltage applied, temperature, input fluid pressure, output fluid pressure, etc. But electromagnetic valves cannot detect and feed back the rate of fluid flow through the valves. Without information on variations in fluid flow through the valve, it is impossible to control the hydraulic pressure with high accuracy by correcting the fluid flow.
A sensor provided in the fluid pressure line would serve this purpose. But providing a separator sensor is an uneconomical solution. Another solution would be to reduce the degree of pressure reduction and pressure increase on each command for finer, more delicate pressure control. But this solution has a problem in that the smaller the degree of pressure increase or reduction on each command, the greater the possibility that the pressure may not rise or lower at all on such command due to variations in the fluid flow through the valve.
Thus, conventional brake fluid pressure control valves have still room for improvement in their ability to carry out pressure increase and reduction commands with high accuracy and quickness.
To solve this problem, unexamined Japanese patent publication 3-90462 proposes a flow control valve 100 shown in FIG. 8. This valve has a piston 102 slidably mounted in a housing 101 to define a first fluid chamber A and a second fluid chamber B between the piston 102 and both ends of the housing 101. The piston 102 is formed with an axial fluid passage 102B including a fixed-area orifice 102A. The passage 102B communicates with an annular groove 102C formed in the outer periphery of the piston.
The piston 102 is biased axially by a spring 103 and an electromagnet 104. The housing 101 has an input port 101A communicating with the fluid passage 102B through the annular groove 102C, and an output port 101B communicating with the second fluid chamber B. This valve further includes a discharge port provided between the annular groove 102C and the second fluid chamber B. But this discharge port is not described any further because it is not necessary for understanding of the present invention.
The fluid flow rate through the orifice 102A is determined by the difference between the pressures at both ends of the orifice 102A, which communicate with the first and second fluid chambers A and B, respectively. This pressure difference is in turn determined by the biasing forces applied to the piston 102 from the spring 103 and the electromagnet 104, respectively.
An orifice variable in flow area (degree of opening) is defined at the intersection of the input port 101A and the annular groove 102C. The degree of opening of this orifice influences the abovementioned pressure difference. That is, when the piston 102 moves rightwardly and the degree of opening of the variable-area orifice increases, the pressure in the first fluid pressure A rises correspondingly, urging the piston 102 leftwardly. Conversely, when the piston 102 is moved leftwardly biased by the electromagnet 104 and the degree of opening of the variable-area orifice decreases, the fluid flow through the variable-area orifice decreases below the fluid flow through the fixed-area orifice 102A. The piston thus begins to move rightwardly biased by the spring 103. (This state is hereinafter referred to as "spontaneous equilibrium state".)
The control force (f-F) applied to the piston 102 during spontaneous equilibrium state is given by: EQU (Pa-Pb)A=f-F (1)
where Pa is the fluid pressure in the first fluid chamber, Pb is the fluid pressure in the second fluid chamber, f is the biasing force of the spring 103, F is the biasing force of the electromagnet 104, and A is the sectional area of the piston 102.
As will be apparent from equation (1), the difference Pa-Pb between the pressures at both ends of the fixed-area orifice 102A is determined solely by the forces f and F and the sectional area A. That is, the fluid pressures at the input port 101A and the output port 101B have no influence on the pressure difference Pa-Pb. The flow rate through the fixed-area orifice 102A is in turn proportional to the square root of the pressure difference. Thus, this flow control valve has the function of controlling the flow independently of the input fluid pressure and output fluid pressure (pressure compensation function).
But this flow control valve cannot prevent an extremely small amount of leakage of hydraulic fluid between the input port 101A and the output port 101B through the sliding surface of the piston 102 indicated by X in FIG. 8.
Thus, this valve is not suitable for use in applications where even an extremely small amount of leakage of hydraulic fluid is not permissible such as the use as a pressure reducing valve in an antilock brake system.
Unexamined Japanese patent publication 6-83457 discloses a flow control valve which is free of the problem of leakage of hydraulic oil while deactivated. As shown in FIG. 9, this valve 200 comprises a housing 211 having an input port 213 and an output port 216, and a piston 212 mounted in the housing 211 so as to be axially slidable in a substantially liquid-tight manner. A first fluid chamber A and a second fluid chamber B are defined between the end faces of the piston 212 and the housing 211. The second fluid chamber B communicates with the output port 216.
The piston 212 is formed with an axial fluid passage 212A including a fixed-area orifice 215 through which the first and second fluid chambers A and B communicate with each other. On its end facing the first fluid chamber A, the piston 212 is provided with a pushrod 212B for opening an on-off valve 214, which is described later. On the end facing the second fluid chamber B, the piston 212 is formed with a piston-driving protrusion 212C.
A spring 218 is mounted in the first fluid chamber A in a compressed state. It biases the piston 212 axially rightwardly to keep it in its initial position. An electromagnet 220 as a second biasing means is mounted in the second fluid chamber B of the housing 211. The piston-driving protrusion 212C is inserted in the electromagnet 220. When a current flows through the electromagnet 220, it biases the piston 212 axially leftwardly in the figure. While no current is flowing through the electromagnet 220, no biasing force is applied to the piston 212 from the electromagnet 220, so that the piston is kept in its initial position shown in FIG. 9 biased by the spring 218.
An on-off valve 214 is disposed between the first port 213 and the first fluid chamber A for opening and closing communication between the first port 213 and the first fluid chamber A. The on-off valve 214 is a puppet valve comprising a seating spring 214A, a spherical valve body 214B, and a valve seat 214C opposite one end of the piston 212 through the first fluid chamber A. Structurally, the puppet valve prevents leakage of fluid from the first port 213 to the second port 216.
Biased rightwardly by the seating spring 214A, the valve body 214B is normally pressed against the valve seat 214C, closing fluid communication between the first port 213 and the first fluid chamber A. When the piston 212 is moved leftwardly in this state, the valve body 214B is pushed leftwardly by the pushrod 212B against the force of the seating spring 214A. This opens fluid communication between the first port 213 and the first fluid chamber A. The flow control valve of this publication further includes a relief valve for bypassing the first and second fluid chambers A, B. But this valve is not described any further because it is not necessary for understanding of the present invention.
In this flow control valve, the piston 212 moves axially leftwardly to open the on-off valve 214 when the following relation (2) is met: EQU Pi.times..alpha.&lt;F-f+Pb.times.A-Pa.times.(A-.alpha.) (2)
where .alpha. is the effective sectional area of the sealing portion of the on-off valve 214, A is the sectional area of the piston 212, Pi is the fluid pressure at the first port 213, Pa is the fluid pressure in the first fluid chamber A, Pb is the fluid pressure in the second fluid chamber B and at the second port 216, f is the biasing force of the spring 218, and F is the biasing force of the electromagnet 220 (the biasing force of the seating spring 214A for keeping the valve body 214B seated is very small and thus ignored in the above formula).
Normally, the relation Pi&gt;Pa.gtoreq.Pb is met. Thus, while the electromagnet 220 is not energized, no force acts on the piston 212 that can overcome the force Pi.times..alpha., so that the fluid communication between the first port 213 and the second port 216 is completely shut off. When the valve 214 is opened by activating the electromagnet, the spontaneous equilibrium state of the piston begins as explained in relation to the flow control valve 100 shown in FIG. 8. But in the case of the flow control valve 200, in order to adjust the flow rate to a desired level, it is necessary to provide a pressure sensor for measuring Pi and Pb at the portions connecting the first port 213 and the second port 216, because the force Pi.times..alpha. is acting in the valve 200.
For example, it is possible to provide the conventional flow control valve 100 with a pressure compensation function by forming in the piston a fixed-area orifice for restricting the fluid flow by use of the difference between the pressures at the input port and the output port and by making use of the spontaneous equilibrium state of the variable-area orifice provided on the outer periphery of the piston. But in this arrangement, it is impossible to prevent fluid leakage along the sliding surface of the piston.
In order to solve this problem, the flow control valve 200 is provided with a puppet valve to prevent leakage of fluid from the input port to the output port. But with this valve, it is necessary to provide a pressure sensor for detecting pressure difference at the input and output ports to feed back the detected pressure difference for the control of the electromagnetic valve in order to eliminate the influence of the fluid pressure at the input port on the puppet valve.
An object of the present invention is to provide a flow control valve which solves all these problems, and in which all the component parts including the fixed-area orifice, variable-area orifice and a puppet valve are provided in the housing.